This invention relates to radio frequency amplifiers and particularly to those utilizing automatic gain control. Such amplifiers are commonly used in the initial amplfication of a received radio frequency signal, particularly in AM receivers.
An RF amplifier which has been utilized in a significant number of radio receivers since its introduction is the dual gate MOSFET, in which the RF signal may be aplied to the lower gate and a gain control voltage applied to the upper gate. The gain controlled output is obtained from the drain of the device. However, although useful, the dual gate MOSFET RF amplifier has limitations in AGC range and overload characteristics.
An improved RF amplifier which may replace a dual gate MOSFET in many circuit applications is a cascode combination of a JFET and bipolar transistor. The drain of the JFET is connected to the emitter of the bipolar transistor; the RF signal is applied to the gate of the JFET; the AGC voltage is applied to the base of the bipolar transistor; and the output is taken from the collector of the bipolar transistor. This amplifier shows superior AGC range and overload characteristics under applied AGC; and, in addition, provides simpler biasing requirement and lower noise than a dual gate MOSFET. However, the bipolar transistor of the combination requires a base current for operation which is considerably greater than the leakage gate current of the upper gate of a dual gate MOSFET. Therefore, if this cascode amplifier is to be substituted directly for a dual gate MOSFET in a circuit application, a larger current drive must be supplied for the bipolar transistor.
In addition, it may be desirable, particularly in some varactor tuned applications, to provide a second stage of RF amplification in order to overcome losses in the tuning circuit without overloading the tuning varactor diodes.